Cones mediate everyday vision, and how cones adapt to bright illumination is an intrinsic part of their function. Despite their importance to vision, little is known about how cones adapt to light. The research proposed here aims to elucidate the molecular mechanisms of light adaptation in cones, and dissect how these mechanisms impact the function and performance of the cone in bright light. The first specific aim determines how cones respond to prolonged background light, focusing on slow changes in circulating current in response to steady illumination. The second specific aim determines how the gain and speed of the photo transduction cascade changes under background light by studying the kinetics of the dim flash response. The third specific aim measures the sensitivity and contribution of specific components of the photo transduction cascade to adaptation. These experiments test how specific mechanisms regulate the gain and speed of the photo transduction cascade during adaptation. A suction electrode technique is used throughout, which allows recording from individual, intact photoreceptors.